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Handicap depth
In the Alexander chess engine, Handicap Depth is a revolutionary feature designed to ensure the engine can truly simulate a human player's thinking system. Traditionally, handicap modes simply force an engine to play random errors, which become more frequent as the set Elo rating decreases. However, this traditional approach is not very useful for practical over-the-board (OTB) players. Instead, Alexander implements a completely different simulation that relies on limiting the engine based on the user's Elo rating.
The correlation between a player's Elo rating and their search depth limit is based on the research of Diego R. Ferreira. Building upon this concept, Alexander utilizes a modern formula to map specific Elo ranges to a restricted handicap search depth. This limitation ensures that the engine does not calculate variations deeper than what a human at that specific skill level could realistically foresee.
The chosen Elo rating defines four traditional Soviet chess schools (or playing levels), which directly influence the analysis depth utilized during the search. By setting a specific Elo, the engine uses the corresponding depth to accurately simulate a human player.
Based on the documentation, the levels and their corresponding handicap depths are defined as follows:
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Beginner:
- Elo Range: [0, 1999]
- Handicap Depth Range: [1, 6] plies
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Intermediate:
- Elo Range: (1999, 2199]
- Handicap Depth Range: (7, 9] plies
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Advanced:
- Elo Range: (2199, 2399]
- Handicap Depth: 10 plies
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Expert:
- Elo Range: 2400 to 2850
- Handicap Depth Range: (11, 20] plies
By halting its calculation once the specified handicap depth is reached, the engine avoids the "brute force" perfection typical of unrestricted machines. When combined with customized evaluation functions for each level, this depth limitation allows Alexander to act as a highly realistic sparring partner tailored to the exact level of the user.